refactor: share Go control-plane client

[steipete]

Summary

• move shared Crabfleet API models, authentication, lifecycle semantics, HTTP calls, and terminal operations into internal/fleetapi
• move terminal-safe user, session-tree, log, status, and summary rendering into internal/fleettext
• keep CLI and SSH gateway entry points focused on command-specific behavior
• make Docker and GitHub Actions track the complete internal/** Go package boundary

Validation

• go test ./...
• go test -race ./...
• go vet ./...
• go build ./cmd/crabfleet ./cmd/crabbox-ssh-gateway
• pnpm check
• pnpm test
• pnpm build
• docker build -f Dockerfile.ssh-gateway .
• autoreview clean after fixing its large JSON response finding

[clawsweeper]

ClawSweeper status: review started.

I am starting a fresh review of this pull request: refactor: share Go control-plane client This is item 1/1 in the current shard. Shard 0/1.

This placeholder means the worker is alive and reading the current context. I will edit this same comment with the actual review when the claws are done clicking.

Crustacean status: shell secured, claws on keyboard, evidence pebbles being sorted.